I would crack the case open and see if the shift fork is gummed up with crud or rust. Assuming it is, a good soak in diesel or PB blaster will help a ton. Clean everything up and regrease. Check your bushings around the shifter for the same. Whenever I get a rider with a stuck shifter as you describe it, that's the problem 9 times out of 10. Might be, although less likely something broken and jammed up against the shift fork or pin.